Let CCACHE_UMASK apply only to files/directories in the cache directory

Let CCACHE_UMASK apply only to files/directories in the cache directory

Ever since the CCACHE_UMASK setting was added in
75dd382407a36b3ebab36bb1027eb43a07498bec it has applied to all files
created by ccache, including files such as the object files and
executables created when ccache is called for linking. This was
presumably the easy thing to do, but it affects the observable output
from the compiler. There are also two related bugs:

1. CCACHE_UMASK is not used when creating the initial configuration file.
2. CCACHE_UMASK is used when creating the log file.

Fix this by:

- saving the original umask in Context,
- introducing a UmaskScope class,
- using UmaskScope objects to set the original umask when calling the
  compiler and when copying result files from the cache, and
- setting the umask after creating the log file but before creating the
  initial configuration file.

Closes #628.