Improve handling of '\' in irpc

[flamewing]

It should probably be detected and replaced by '\' in most cases. This will reduce the need for stupid workarounds like this:

vtputs macro letters,clr
    irpc char,letters
        if "char\t"=="\\t"    ; <- detect backslash
            vtputc  "\\",clr
        else
            vtputc  "char",clr
        endif
    endm
    endm