src/bin/grepc{,_c,_mk}: Split grepc(1) into helper programs

src/bin/grepc{,_c,_mk}: Split grepc(1) into helper programs

These programs print the PCRE2 patterns used by grepc(1).  This allows
easily implementing different drivers for parsing different languages.

It also allows easily using pcre2grep(1) in cases where grepc(1) might
be limited; for example, searching in non-regular files, or passing
other flags that grepc(1) doesn't pass to pcre2grep(1).

Signed-off-by: Alejandro Colomar <alx@kernel.org>