devel/libfixposix: Import upstream fix

Changes `lfp_sendfile()' so it mimics Linux sendfile(2), returning on
success the number of bytes, instead of just 0. While here, adopt the
port.

PR: 260181
Obtained from: https://github.com/sionescu/libfixposix/commit/fc5bbe41a574e668ccfafb0f15dddd28f6decd31
Sponsored by: Rubicon Communications, LLC ("Netgate")