Implement fork handling for keysinuse context management

[jasjivsingh-corp]

These changes are improvement on #149

Changes

• Added keysinuse_atfork_prepare() and keysinuse_atfork_parent() handlers
• Added helper functions keysinuse_lock_key_ctx() and keysinuse_unlock_key_ctx()
• Simplified keysinuse_atfork_reinit() - no longer needs to handle locking since prepare already acquired all locks
• Simplified keysinuse_reset_key_ctx() - lock recreation moved after counter reset

Testing

• Tested with nginx worker process forking
• Ran KeysInUseTest

[mamckee]

static void keysinuse_reset_key_ctx(SCOSSL_KEYSINUSE_CTX_IMP *ctx)

Maybe rename this to keysinuse_atfork_reset_key_ctx so it's clear this is only to be used in the atfork #Resolved

---

Refers to: KeysInUse/keysinuse.c:812 in b0a136e. [](commit_id = b0a136e, deletion_comment = False)

[mamckee]

LGTM once the comments are addressed.

[mamckee]