style.9: update C99 commentary

Make style.9 read as a current statement of C99 preferences, rather than a
description of ongoing changes to our preferred style.  Alsu use the short
form "ISO C99" on the 2nd and later instances rather than repeating the
unwieldy `ISO/IEC 9899:1999 ("ISO C99")` each time.

Reviewed by:	cem, imp, jhb, kib
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D23648

share/man/man9/style.9

@@ -301,11 +301,11 @@ is treated as
 #endif /* !COMPAT_43 */
 .Ed
 .Pp
-The project is slowly moving to use the
+The project prefers the use of
 .St -isoC-99
 unsigned integer identifiers of the form
 .Vt uintXX_t
-in preference to the older
+rather than the older
 .Bx Ns -style
 integer identifiers of the form
 .Vt u_intXX_t .
@@ -317,10 +317,10 @@ Like white-space commits, care should be taken in making
 .Vt uintXX_t
 only commits.
 .Pp
-Similarly, the project is slowly moving to use the
-.St -isoC-99
+Similarly, the project prefers the use of
+ISO C99
 .Vt bool
-in preference to the older
+rather than the older
 .Vt int
 or
 .Vt boolean_t .
@@ -341,8 +341,8 @@ Userspace code should include
 while kernel code should include
 .In sys/types.h .
 .Pp
-Likewise, the project is moving to using the
-.St -isoC-99
+Likewise, the project prefers
+ISO C99
 designated initializers when it makes sense to do so.
 .Pp
 Enumeration values are all uppercase.