Import grepc(1) (script and manual page) from separate project

It started as a script in the Linux man-pages project, under scripts/
within scripts/bash_aliases.

Eventually, I turned it into a large script, and moved it into a
separate project, to be able to work on it more freely.

The original scripts were removed from this project in commit
c3752af0549c (2022-11-22; "bash_aliases: Remove grep_syscall(),
grep_syscall_def(), and grep_glibc_prototype()").

Now, grepc(1) has stabilized, and I find it very useful for maintaining
this project (as well as others), so let's merge it back.  This will
allow distros to package it sooner than if it remained as a standalone
project.

I've imported the git history (ignoring files we don't care about) with
the help of git-filter-repo(1).

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