utempter: relaxed host argument validation: it is now allowed

Removed obsolete FreeBSD support.
Enabled build with non-glibc libcs.
Silenced -Wconversion and -Wstringop-truncation compilation warnings.
Added deprecation warnings to the old API.
Implemented host argument validation.
Implemented optional syslog support.
Built with -Werror.

Parametrized compiler and linker options further.
Built utempter as a PIE.

Rebuilt for debuginfo.

Added manpages documenting the utempter interface, based on

Removed obsolete %post_ldconfig/%postun_ldconfig calls.
Removed redundant dependencies.

Removed cvsid tags.
README: Described differences between provided libutempter interfaces.
Fixed FreeBSD support:
glibc systems too.

Enabled almost all diagnostics supported by gcc and fixed all
3.4.4-alt3.
Added FreeBSD support, based on patches from Gentoo/FreeBSD.
Makefile:
libutempter: Linked with -Wl,-z,defs.
utempter:
bit architectures
bit backwards compatibility enabled (like x86_64).
Wl,-z,now, i.e., marked it to tell the dynamic

Restricted list of global symbols exported by the library.
Updated FSF postal address.

Added multilib support.

iface.c: don't block SIGCHLD; redefine signal handler instead.

Changed soname back to libutempter.so.0, introduced versioning.

If helper execution fails, try saved group ID.

New function: utempter_set_helper.

iface.c: block SIGCHLD instead of redefine signal handler.

utempter.h: do not use "__attribute ((unused))".