config: use std::string in md_config_t

config: use std::string in md_config_t

Use std::string to represent md_config_t strings. This makes memory
management a lot easier and should fix some leaks. "No value" is now
represented by an empty string, whereas before some places were using
empty strings and some were using NULL.

config.cc: Fix a minor decode bug.

In pid_file.cc, copy the pid_file using snprintf, since strncpy
does not always NULL-terminate.

Signed-off-by: Colin McCabe <colin.mccabe@dreamhost.com>