libpkg: make shlib handling portable

All shared objects NEEDED by an ELF file are added to shlibs_required
by pkg-create rather than filtering them based on the host system ELF
hints file and installed libraries.

The ALLOW_BASE_SHLIBS option is deprecated and ignored.

Before running the solver, pkg now scans /lib and /usr/lib (taking
--rootdir into account) and builds a list of system-provided shlibs that
is used as an input to the solver. If pkg detects that it is targeting a
pkgbase system this scan for system shlibs is skipped as the base
packages will provide all necessary shlibs.

The detection of a pkgbase system is implemented by checking if
/usr/bin/uname is tracked in the pkg database.

The main user-facing change is that pkg-create now adds *all* shared
objects listed as NEEDED by an ELF file to shlibs_required. In general
this should not cause issues due to the solver taking the shlibs
provided by the base system into account but there may be exceptions.
An option to filter these entries will be added in the next commit.

References:	freebsd#2331
Sponsored by:	The FreeBSD Foundation

libpkg/Makefile.autosetup

@@ -36,7 +36,8 @@ SRCS=	backup_lib.c \
 	pkg_elf.c \
 	pkg_abi_macho.c \
 	binfmt_macho.c \
-	ssh.c elfhints.c \
+	ssh.c \
+	system_shlibs.c \
 	pkg_arch.c \
 	pkg_cudf.c \
 	pkg_jobs_universe.c  pkg_printf.c \