mask.c: Allow using static placeholders within custom placeholders
?1..?4.

# three-letter-words, capitalized or not
$ ../run/john -4=?u?l -mask=?4?l?l -stdout

# hex digits 0x00-0xff
$ ../run/john -4=[?da-f] -mask=0x?4?4 -stdout

# test case, watch escaped characters
$ ../run/john -stdout -enc:ascii -1=a-z -2=[A-Z] -3=?u?l -4=[?d?s] -mask=?1?2?3?4