scripts/bash_aliases: Make man_lsfunc() more robust; Add sed_rm_ccomments().

scripts/bash_aliases: Make man_lsfunc() more robust; Add sed_rm_ccomments().

This patch makes man_lsfunc() search for the function prototypes,
instead of relying on the current manual page formatting,
which might change in the future, and break this function.

It also simplifies the code, by reusing man_section().

Create a new function sed_rm_ccomments(), which is needed by
man_lsfunc(), and may also be useful in other cases.

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>