locks: add configuration and command-line options to enable/disable locks (#7692)

- spack.util.lock behaves the same as llnl.util.lock, but Lock._lock and
  Lock._unlock do nothing.

- can be disabled with a control variable.

- configuration options can enable/disable locking:
  - `locks` option in spack configuration controls whether Spack will use filesystem locks or not.
  - `-l` and `-L` command-line options can force-disable or force-enable locking.

- Spack will check for group- and world-writability before disabling
  locks, and it will not allow a group- or world-writable instance to
  have locks disabled.

- update documentation

etc/spack/defaults/config.yaml

@@ -18,13 +18,16 @@ config:
   # You can use $spack here to refer to the root of the spack instance.
   install_tree: $spack/opt/spack
 
+
   # Locations where templates should be found
   template_dirs:
     - $spack/templates
 
+
   # default directory layout
   directory_layout: "${ARCHITECTURE}/${COMPILERNAME}-${COMPILERVER}/${PACKAGE}-${VERSION}-${HASH}"
 
+
   # Locations where different types of modules should be installed.
   module_roots:
     tcl:    $spack/share/spack/modules
@@ -74,6 +77,15 @@ config:
   dirty: false
 
 
+  # When set to true, concurrent instances of Spack will use locks to
+  # avoid modifying the install tree, database file, etc. If false, Spack
+  # will disable all locking, but you must NOT run concurrent instances
+  # of Spack.  For filesystems that don't support locking, you should set
+  # this to false and run one Spack at a time, but otherwise we recommend
+  # enabling locks.
+  locks: true
+
+
   # The default number of jobs to use when running `make` in parallel.
   # If set to 4, for example, `spack install` will run `make -j4`.
   # If not set, all available cores are used by default.