unwind: refactor stacktrace_walk

* unwind.c (stacktrace_walk): Move stack frame printing code
to separate function print_stack_frame.
unwind: remove unused field from mmap_cache_t

* unwind.c (mmap_cache_t): Remove "deleted" field.
(build_mmap_cache): Remove initialization of "deleted" field.
unwind: cleanup build_mmap_cache

* unwind.c (build_mmap_cache): Move local variables to the code branch
where they are used.  Check return code of sscanf and strdup.  Do not
treat unusual memory mappings as fatal errors.  Do not skip memory
mappings with path names starting with "[".
unwind: ignore memory mappings that have no PROT_EXEC bit set

* unwind.c (build_mmap_cache): For each memory mapping being scanned,
save its PROT_EXEC bit and skip the mapping if it is not set.
tests: enhance -k test

Add two more function calls to the stack.  Suggested by Masatake YAMATO.

* tests/stack-fcall.c (f1): Rename to f3.
(f1, f2): New functions.
* tests/strace-k.test: Update.
tests: robustify -k test

Split stack-fcall.c into several compilation units so that intermediate
function calls would not be optimized out by compiler.

* tests/stack-fcall.c: Move intermediate functions to ...
* tests/stack-fcall-*.c: ... new files.
* tests/Makefile.am (stack_fcall_SOURCES): Add stack-fcall-*.c.
Document -k option as experimental

strace -k does not produce a reliable output on all supported
configurations yet, even basic strace-k.test is known to fail
on some of them.

* strace.c (usage): Document -k option as experimental.
* strace.1: Likewise.
* NEWS: Likewise.
xtensa: sort values in struct_user_offsets

Otherwise ptrace syscall argument decoding is wrong:
  ptrace(PTRACE_PEEKUSER, 296, syscall_nr, [0x4048eb]) = 0
  ptrace(PTRACE_PEEKUSER, 296, syscall_nr, [0x3fa6cd30]) = 0
  ptrace(PTRACE_PEEKUSER, 296, syscall_nr, [0x8040676d]) = 0
instead of
  ptrace(PTRACE_PEEKUSER, 296, pc, [0x4048eb]) = 0
  ptrace(PTRACE_PEEKUSER, 296, a1, [0x3fa6cd30]) = 0
  ptrace(PTRACE_PEEKUSER, 296, a0, [0x8040676d]) = 0

* process.c (struct_user_offsets) [XTENSA]: Sort values.