crond: tweak help text, make course less cryptic

function                                             old     new   delta
packed_usage                                       29736   29740      +4
crond_main                                          1485    1486      +1
load_crontab                                         712     706      -6

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
libbb: make syslog level for bb_error_msg's configurable. use it in crond

function                                             old     new   delta
bb_verror_msg                                        380     386      +6
syslog_level                                           -       1      +1
crondlog                                             165     108     -57

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
crond: simplify logging code

Slight change in behavior: now -l 5 will enable the same messages
formerly only enabled for -d 5. (Old behavior was non-sensical).

function                                             old     new   delta
reopen_logfile_to_stderr                               -      34     +34
log8                                                   -      26     +26
log7                                                   -      26     +26
log5                                                   -      26     +26
load_crontab                                         706     688     -18
rescan_crontab_dir                                   157     137     -20
crond_main                                          1486    1456     -30
fork_job                                             389     343     -46
crondlog                                             108      38     -70
------------------------------------------------------------------------------
(add/remove: 4/0 grow/shrink: 0/5 up/down: 112/-184)          Total: -72 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
crond: implement "SHELL=prog" in crontab. Export LOGNAME as POSIX wants

function                                             old     new   delta
fork_job                                             343     456    +113
load_crontab                                         688     777     +89
crond_main                                          1456    1434     -22
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 202/-22)           Total: 180 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>