man: don't skip default path which appears in config file

man: don't skip default path which appears in config file

If the MANPATH environment variable isn't set a provisional default
path of /usr/man is placed in man_path_list.  This is only used if a
configuration file doesn't contain an alternative path.

If a configuration file lists the default path first:

   MANPATH /usr/man:/usr/share/man

add_MANPATH() sees that the default entry is already present and skips
it.  As a result man_path_list only contains the second and subsequent
components of the configured MANPATH.

In such cases the path should not be skipped.

function                                             old     new   delta
add_MANPATH                                          170     183     +13
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(add/remove: 0/0 grow/shrink: 1/0 up/down: 13/0)               Total: 13 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>