mp(2): fix mpsignif, mplowbits0
 [rsc] --rw-rw-r-- M 1138436 glenda sys 10762 Feb 16 11:18 sys/man/2/mp
	/n/sourcesdump/2006/0216/plan9/sys/man/2/mp:479,493 - 
	/n/sourcesdump/2006/0217/plan9/sys/man/2/mp:479,499
	  .IR m .
	  .PP
	  .I Mpsignif
	- returns the bit offset of the left most 1 bit in
	+ returns the number of significant bits in
	  .IR b .
	  .I Mplowbits0
	- returns the bit offset of the right most 1 bit.
	+ returns the number of consecutive zero bits
	+ at the low end of the significant bits.
	  For example, for 0x14,
	  .I mpsignif
	- would return 4 and
	+ returns 4 and
	  .I mplowbits0
	- would return 2.
	+ returns 2.
	+ For 0, 
	+ .I mpsignif
	+ and
	+ .I mplowbits0
	+ both return 0.
	  .PP
	  The remaining routines all work on arrays of
	  .B mpdigit
 [rsc] --rw-rw-r-- M 1138436 glenda sys 10762 Feb 16 11:18 sys/man/2/mp