linux: spl: generic: ddi_strto*: match solaris ddi_strto*(9)

Recognise initial whitespace, + in both cases,
and - also in unsigneds

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13434