Document multi-component profile paths

In env_variables.rst and krb5_conf.rst, document that KRB5_CONFIG can
contain multiple colon-separated pathnames.

ticket: 8031 (new)
target_version: 1.13.1
tags: pullup
Simplify prof_parse.c include support

We do not need to pass the whole parser state to parse_include_file
and parse_include_dir, only the root section.  Also constify the
filename and dirname parameters.
Add support for directories in profile paths

If a profile path component is a directory, process files in the
directory as we would for an "includedir" directive.

[ghudson@mit.edu: don't change default profile path; simplify
profile_process_directory using prior commit; only check stat bits,
not final character of pathname; misc style changes; commit message]

ticket: 8030 (new)
Add test case for directory in profile path

ticket: 8030
Document directory names in profile paths

In krb5_conf.rst, document that KRB5_CONFIG can contain directory
names.

ticket: 8030