Allow NSH date command with no RTC.  This command is useful without an RTC too.  Also, this permits testing on the simulator which never has an RTC

Fixes to asinh(), atanh(), and sinh(): The 'basic' expansions all exhibited bad cancellation errors near zero (<= 1E-10). This can be easily seen e.g. with x = 1E-30, the results are all zero though they should be extremely close to x. The cutoff values (1E-5, 1E-9) are chosen so that the next term in the Taylor series is negligible (for double). Functions could maybe be optimized to use only first term (x) and a smaller cutoff, just bigger than where the cancellation occurs.

SAM3/4 and SAMV7 UART:  The IMR register is read-only.  This means that sam_restoreints() does not actually re-enable UART interrupts.

A side-effect of changing serial settings via TERMIOS (such as tcsetattr) is that serial interrupts were being left disabled.  This is not a problem if the serial configuration is changed when there are no open references to the serial device.  In that case, serial interrupts are disabled and will not be enabled enabled until the serial device is first opened.  But it is fatal if the serial device is already opened and if there is a task waiting to receive data.  In that case, the side-effect of disabling interrupts is fatal:  That task is then left hanging with interrupts disabled.