Fix readline/editline support

[memreflect]

Configuration bugs fixed:

• --without-readline would set use_readline=yes when it should have set use_readline=no.
• --with-editline correctly found libedit, but HAVE_LIBEDIT was consequently defined rather than HAVE_LIBEDITLINE, resulting in a useless dependency on libedit since there was no readline functionality available.

Slightly altered behavior of --with-readline and --with-editline to result in an error when a readline library was explicitly requested:

• If neither are specified, readline support from libreadline will be used if found. No error will occur if it is not found. This maintains existing behavior.
• --without-readline will disable libreadline, even if it would ordinarily be used. This maintains intended behavior.
• --with-editline will try to find readline support in libedit first, then libreadline unless --with/without-readline was specified. If neither is found, an error will occur.
• --with-readline and --with-editline both specified will try to find readline support in libreadline first, then libedit. This maintains existing behavior. If neither is found, an error will occur.

Other changes:

• HAVE_READLINE is now defined in config.h instead of the separate HAVE_LIBREADLINE and HAVE_LIBEDITLINE. The first detected library will be linked automatically.
• libtinfo is now searched. Linux and FreeBSD, for example, use libtinfo rather than libterminfo, so the search order for terminal capability libraries is terminfo, then tinfo, and finally termcap.
• HAVE_LIBTERMCAP and HAVE_LIBTERMINFO in config.h are gone. Previously, AC_CHECK_LIB would define them, but the switch to AC_SEARCH_LIBS means they are no longer AC_DEFINE'd implicitly if found. Since the library is only needed at link time, and there is nothing in the source code relying on them, there is no need to define a preprocessor symbol, thus they were not replaced.
• AC_PREREQ([2.58]) added since AS_HELP_STRING is used.

[memreflect]

Closed due to a misunderstanding. It enabled compilation with libedit while incorrectly disabling libeditline support, which I initially thought were the same since the editline.shar file in the README compiles as "libedit".