- Rewrote the env var management code with Vixie cron compatibility in mind

- Rewrote the env var management code with Vixie cron compatibility in mind
 * new env_list_t type with associated functions
 * build some char **envp arrays and pass them to execle()
 * cleaner management of env var defined several times
 * don't inherit fcron's environment when running a job but build the environment from scratch
 * list the env var as X-Cron-Env headers in the job output emails
 * add Auto-Submitted headers to the job output emails (as cronie, i.e. cron on RedHat)
 * added a default path for jobs: /usr/bin:/bin (as Vixie cron)
 * ignore not only USER but LOGNAME assigments as well
 * added my_unsetenv() and my_setenv_overwrite() to make the direct manipulation (without env_list_t) of env var easier
- no longer try to run a job with execlp("sh", ...) if execl(curshell, ...) fails (as vixie cron)
- improved u_list: realloc() instead of free() followed by a calloc()
- debug_opt is now defined by log.c (instead of fcron.c/fcrontab.c/...)