Add .library("lib").mark("mark") and use it for .mark semaphores.

Add .library("lib").mark("mark") and use it for .mark semaphores.

tapset-utrace.cxx (TOK_LIBRARY): New.
 (utrace_derived_probe::utrace_derived_probe): Add library and has_library.
 (utrace_builder::build): Handle library.
 (utrace_derived_probe_group::emit_probe_decl): Add sdt_sem_offset to
  emitted stap_utrace_probes.  Add stap_task_finder_target
  mmap_callback for handling shared library.  Handle sdt_sem_offset in
  emitted_stp_utrace_probe_cb.  Add stap_utrace_mmap_found.
 (register_tapset_utrace): Handle .library
tapset-utrace.cxx (TOK_LIBRARY): New.
 (base_query::base_query):  Add path and has_library.
 (dwarf_derived_probe::dwarf_derived_probe) Likewise.
 (dwarf_derived_probe::register_patterns):  Handle .library
 (sdt_query::convert_location): Likewise.
 (dwarf_builder::build):  Likewise.
 (uprobe_derived_probe_group::emit_module_decls): Emit
  sdt_sem_address.  Add sdt_sem_offset to emitted stap_uprobe_spec.
  Add offset and vm_flags to signature of stap_uprobe_change_plus, and
  handle sdt_sem_offset.  Allow writeable segments in emitted
  stap_uprobe_mmap_found.
sdt_misc.exp:  Test .library
util.cxx (find_executable):  Add env_path to sig and use it in getenv.
util.h (find_executable): Likewise.  Make "PATH" the default.
dtrace.in (provider): Turn on semaphores.
sdt.h: Likewise.