Fix floating point numerics having precision of 0 with assignments (#149)

Issuing typeset floating point numerics having a precision of 0
failed as the precision/size was being overwritten with the string
length of the value, e.g. 'typeset -F0 x=5.67' would result in
'typeset -F 4 x=5.6700' as len('5.67') is 4.

src/cmd/ksh93/include/nval.h:

- Created a symbolic name of NV_FLTSIZEZERO to respresent a float
  having a precision/size of 0. NV_FLTSIZEZERO needs to be a
  negative value.

src/cmd/ksh93/bltins/typeset.c:

- In b_typeset(), added code to set tdata.argnum to NV_FLTSIZEZERO
  for E, F, X options.

- In setall(), adjusted code to allow for tp->argnum to be negative.

src/cmd/ksh93/sh/name.c: nv_newattr():

- Adjusted option value only change code to handle NV_FLTSIZEZERO as
  well as changed to directly setting np->nvsize instead of using
  nv_setsize(np,size) as nv_setsize might contain conflicting and/or
  redundant code.

- Added missing conditional check of '!(newatts&NV_INTEGER)' to
  constrain the size==0 code block to justified strings as
  NV_LJUST, NV_RJUST, or NV_ZFILL are only valid for strings if
  NV_INTEGER is not set. This code block was mistakenly setting
  the precision/size value to the length of the value of an
  assignment for floats whereas it should only be performing
  auto assignment length for justified strings.

src/cmd/ksh93/include/nval.h

@@ -165,6 +165,7 @@ struct Namval
 #define NV_DOUBLE	(NV_INTEGER|NV_ZFILL)	/* for floating point */
 #define NV_EXPNOTE	(NV_LJUST)	/* for scientific notation */
 #define NV_HEXFLOAT	(NV_LTOU)	/* for C99 base16 float notation */
+#define NV_FLTSIZEZERO	-1		/* a float with size of 0 being <0 */
 
 /*  options for nv_open */