lib: remove broken Int::sin/cos

Not only these functions weren't available on Android, but their
usage was invalid.

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: in Int::sqrt, use C sqrt function for double

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: move Passwd::gecos to a new posix_ext module

The C function is only provided on some implementation of POSIX and
absent from any POSIX version.

See GNU man page of getpwnam #Conforming to.

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
examples: simplify Makefiles of mnit examples

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: add doc to Passwd properties

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>