This CL fixes a long-lasting bug that prevented pprof from recognizing
Legacy heap profile produced by Go. Go reports four types of samples
at once so the profile includes alloc_objects, alloc_space,
inuse_objects, and inuse_space. The bug caused pprof to misclassify Go
heap profile data and prevent selection of correct filtering/pruning
patterns in analysis.

Update golang/go#25096

Tested with the profile samples included in golang.org/issues/25096
(pprof hides the runtime functions with this change)

In addition to the "goroutine" and "threadcreate" profiles,
Go code can generate custom profiles using the runtime/pprof package.
The user must name these profiles, and the docs recommend using the
convention "import/path" to avoid namespace conflicts.  This CL
updates the pprof tool to be able to parse legacy profiles whose types
contain slashes and other non-space characters.

This is the upstream fix for https://github.com/golang/go/issues/13195.
This change will need to be mirrored to
github.com/golang/go/src/cmd/pprof/internal/profile/legacy_profile.go

A previous commit attempted to handler mappings produces using the glog
package by matching the column on which the sentinel was found.

However, some tools generate the mapping using a single log entry,
where the prefix appears only on the first line.

Instead of matching by column, use a regexp to identify and match
prefixes introduced by the glog package.

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.

Also update the build instructions to use go get