If the memory map is generated by logging routines such as glog, they
may include some initial text which confuses the parsing of legacy
mappings. The initial text is the same for all mapping entries,
so detect it on the proc map sentinel and remove it from the mapping
entries.

Entry mappings of the form:
 "  02e00000-02e8a000: /foo/bin (deleted)"
are currently resulting in "(deleted)" being set as the build id.
Enforce the build id to be hex, and allow unrecognized junk to
be at the end of the mapping.

Simplify regexps and define a recommended format:
  Start   End     object file name     offset(optional)   linker build id
  0x40000-0x80000 /path/to/binary      (@FF00)            abc123456

Also include some additional tests and move existing tests to legacy_profile_test.go,
closer to the code in legacy_profile.go.

This uses https://go-review.googlesource.com/#/c/33635 instead of the
previous change to fix https://github.com/google/pprof/issues/54.

See https://github.com/google/pprof/issues/54. Frames under
call32/call64 may be user code frames so should be shown to avoid
confusing re-attribution of allocations to the calling system frames.

A previous commit tried to eliminate a panic when reading an empty
profile by stopping support for profiles with no samples. That is
unnecessary and it can obstruct some testing.

The panic was caused by the legacy parser returning a nil profile and
no error when processing an empty profile. Detect that situation and
generate a proper profile instead.

Tested by running 'pprof /dev/null'
Add testcase for profile parsing errors

Preserving the old ones while needed

Should have no functional changes

- Remove dead code, unnecessary conversions
- Consistently use a single space after period

Legacy profiles that are not based on interrupt-based sampling
only include addresses that are call stack return addresses. For
them, all callstack addresses should be adjusted by -1 to land
on top of the call instruction and permit accurate symbolization.

This fixes issue #9

The legacy profilez parser handles duplicate leaf samples that are a
common artifact of satck unwinding. Apply the same technique to threadz
profiles where duplicate samples also occur.

A Go redefinition error causes pprof to skip parsing profile mappings.

Also update the build instructions to use go get