Move path_simplify_and_warn() to new shared/parse-helpers.c

Move path_simplify_and_warn() to new shared/parse-helpers.c

This is a high-level function, and it belongs in libsystemd-shared. This way we
don't end up linking a separate copy into various binaries. It would even end
up in libsystemd, where it is not needed. (Maybe it'd be removed in some
optimization phase, but it's better to not rely on that.)

$ grep -l -r -a 'path is not absolute%s' build/
build/libnss_systemd.so.2
build/pam_systemd_home.so
build/test-dlopen
build/src/basic/libbasic.a.p/path-util.c.o
build/src/basic/libbasic.a
build/src/shared/libsystemd-shared-249.so
build/test-bus-error
build/libnss_mymachines.so.2
build/pam_systemd.so
build/libnss_resolve.so.2
build/libnss_myhostname.so.2
build/libsystemd.so.0.32.0
build/libudev.so.1.7.2

$ grep -l -r -a 'path is not absolute%s' build/
build/src/shared/libsystemd-shared-251.a.p/parse-helpers.c.o
build/src/shared/libsystemd-shared-251.a
build/src/shared/libsystemd-shared-251.so

No functional change.