Refactor: fencer: Best practices in get_stonith_flag()

• Add Doxygen.
• Return an enum value rather than a uint64_t value.
• Rename to fenced_parse_notify_flag() in accordance with our current convention for flag parsing functions.
• Rename "name" argument to "type".
• Replace "else if" with "if" where possible.
• Match case-sensitively. STONITH_OP_DEVICE_ADD and STONITH_OP_DEVICE_DEL may be coming from an arbitrary request. However, if the user is making the request via the stonith API functions (as any sane user should be), then the case will always match exactly.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>