Split predefined `$variables` into categories

[XVilka]

[0x00006b10]> %$?
Usage: %$ [<var>]   
| flag          offset of flag
| ${ev}         get value of eval config variable
| $$            here (current virtual seek)
| $$$           current non-temporary virtual seek
| $?            last comparison value
| $alias=value  alias commands (simple macros)
| $B            base address (aligned lowest map address)
| $b            block size
| $c            get terminal width in character columns
| $Cn           get nth call of function
| $D            current debug map base address ?v $D @ rsp
| $DB           same as dbg.baddr, progam base address
| $DD           current debug map size
| $Dn           get nth data reference in function
| $e            1 if end of block, else 0
| $e{flag}      end of flag (flag->offset + flag->size)
| $f            jump fail address (e.g. jz 0x10 => next instruction)
| $F            Same as $FB
| $Fb           begin of basic block
| $FB           begin of function
| $Fe           end of basic block
| $FE           end of function
| $Ff           function false destination
| $Fi           basic block instructions
| $FI           function instructions
| $Fj           function jump destination
| $fl           flag length (size) at current address (fla; pD $l @ entry0)
| $FS           function size (linear length)
| $Fs           size of the current basic block
| $FSS          function size (sum bb sizes)
| $j            jump address (e.g. jmp 0x10, jz 0x10 => 0x10)
| $Ja           get nth jump of function
| $k{kv}        get value of an sdb query value
| $l            opcode length
| $M            map address (lowest map address)
| $m            opcode memory reference (e.g. mov eax,[0x10] => 0x10)
| $MM           map size (lowest map address)
| $O            cursor here (current offset pointed by the cursor)
| $o            here (current disk io offset)
| $p            getpid()
| $P            pid of children (only in debug)
| $r            get console height (in rows, see $c for columns)
| $r{reg}       get value of named register
| $s            file size
| $S            section offset
| $SS           section size
| $s{flag}      get size of flag
| $v            opcode immediate value (e.g. lui a0,0x8010 => 0x8010)
| $w            get word size, 4 if asm.bits=32, 8 if 64, ...
| $Xn           get nth xref of function

We could make the help better by splitting those into categories. Moreover, some of the variables could probably be renamed for clarity and consistency.

[ret2libc]

This needs some more thinking as $ is also reserved for aliases, so having for example an alias $c could be quite confusing.

[XVilka]

@ret2libc maybe we could use %var% syntax, like Windows cmd.exe?

[ret2libc]

That would mix a bit with % (env) commands... But since % is already the same as env we could just use env and allocate % for the variables.