install_man should allow defining additional symbolic links

[kevemueller]

Many complex libraries create general man pages for a set of functions and symlink all function name man pages to that general man. page.
Currently I need to install_man the actual man page and then run a loop over a dict to create the symlinks:

MAN=['archive_entry.3', ...]
MLINKS = {
    'archive_entry.3': [
        'archive_entry_clear.3',
        'archive_entry_clone.3',
        'archive_entry_free.3',
        'archive_entry_new.3',
    ],
    'archive_entry_acl.3': [ .... ]
}

install_man(MAN)
foreach MAN, MLINKG : MLINKS
    foreach MLINK : MLINKG
        install_symlink(
            MLINK,
            install_dir: get_option('mandir') / 'man3',
            pointing_to: MAN,
        )
    endforeach
endforeach

This is a lot of boilerplate code, moreover it is error prone. install_man has heuristics to determine the directory of the manual page. This needs to be replicated when manually creating the symlink as install_man does not return the actual paths of the placed manual page(s).

Instead install_man should allow passing a dict directly and install both the manual page as well as all symlinks.

Second best would be to return the path of the installed manual page(s) in install_man. The code would then be at least not trying to guess.

foreach MAN, MLINKG : MLINKS
    MANT = install_man(MAN)
    foreach MLINK : MLINKG
        install_symlink(
            MLINK,
            install_dir: fs.parent(MANT),
            pointing_to: fs.name(MANT),
        )
    endforeach
endforeach