MEDIUM: ssl: Add local passphrase cache

MEDIUM: ssl: Add local passphrase cache

Instead of calling the external password command for all loaded
encrypted certificates, we will keep a local password cache.
The passwords won't be stored as plain text, they will be stored
obfuscated into the password cache. The obfuscation is simply based on a
XOR'ing with a random number built during init.
After init is performed, the password cache is overwritten and freed so
that no dangling info allowing to dump the passwords remains.