Support passing auto as the value to the JULIA_NUM_THREADS environment variable

[DilumAluthge]

With this pull request, if the JULIA_NUM_THREADS environment variable is set to auto, then the number of threads is set automatically to the number of CPU threads. This is similar to the behavior of the --threads=auto command-line flag.

Example usage:

$ export JULIA_NUM_THREADS="auto"
$ ./julia
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.7.0-DEV.131 (2020-12-20)
 _/ |\__'_|_|_|\__'_|  |  dpa/julia-num-threads-env-auto/96bc22f8c2 (fork: 1 commits, 0 days)
|__/                   |

julia> Threads.nthreads()
4

julia> ENV["JULIA_NUM_THREADS"]
"auto"

[vtjnash]

Landed in 684d5fc (with style nits fixed)