Set session name from environment variable for user commands

Default session name is taken from COPYQ_SESSION_NAME enviroment
variable. Arguments for "copyq" in user commands don't have to be
modified and user can still override session name from command line.
COPYQ_LOG_LEVEL env variable to control log verbosity

Set COPYQ_LOG_LEVEL environment variable to control verbosity of
messages printed on stderr.

COPYQ_LOG_LEVEL can be one of following:

    TRACE   - print trace messages (very verbose debug)
    DEBUG   - print debug messages, default for debug build
    NOTE    - print notes, default for release build
    WARNING - print warnings
    ERROR   - print errors

If COPYQ_LOG_LEVEL is set to a level, all log levels below that level
are also printed. For example, if COPYQ_LOG_LEVEL=NOTE, messages for
NOTE, WARNING and ERROR log level are printed.
COPYQ_LOG_FILE env variable for log file name

If COPYQ_LOG_FILE environment variable is set, its value is used as file
name for logging instead of stderr.

Using this variable allows the monitor process to use the log file too.