Use strings.Builder

It's been there since Go 1.10 and go.mod requires at least Go 1.12.

Fix the worker concurrency model (google#126)

Limit the number of concurrent workers by starting a fixed number of goroutines and providing work by an input channel.

Fixes google#123

README.md: fix typo (google#119)

internal: use filepath.Join in PIDFile (google#114)

Use filepath.Join to create a path with correct path delimiters on
Windows.

Based on a change by @42wim submitted in google#107

agent: check error return values (google#109)

In func handle, check the error return values of pprof.WriteHeapProfile
and trace.Start.

Add prefix to profile dump file names (google#101)

When getting profile dumps for further processing, it's hard to distinguish
CPU and heap profiles from the file names alone. Make this easier by
adding a prefix (cpu or heap) to the file name depending on the type of
profile.

Improve the help text (google#94)

Avoid using tabs, remove help from "subcommands" list,
don't differentiate the profiling commands from others.

Add Go modules and missing vendored file

fix out of bounds panic

vendor: dep ensure -update github.com/shirou/gopsutil (google#79)

gopsutil library is licensed under BSD, but the previous version
contained some GPL-licensed code which is a violation of GPL
license. The updated version doesn't contain GPL-licensed code
anymore.

shirou/gopsutil#518