strfmon: Fix an edge case when sep_by_space is 2

strfmon: Fix an edge case when sep_by_space is 2

Fix an edge case by printing the required space when, the currency
symbol succeeds the value, a space separates the sign from the value and
the sign position precedes the quantity and the currency symbol.

In other words:

    n_cs_precedes = 0
    n_sep_by_space = 2
    n_sign_posn = 1

From The Open Group's localeconv[1]:

> When {p,n,int_p,int_n}_sep_by_space is 2:
> If the currency symbol and sign string are adjacent, a space separates
> them; otherwise, a space separates the sign string from the value.

    Format    Before        After
    [%n]      [-123.45¤]    [- 123.45¤]

[1]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/localeconv.html

Obtained from:	Darwin
Reviewed by:	kib
PR:	267282
Github PR:	#619
MFC after:	1 week