manpages: consistent syntax

In the manpages, bold is used to denote characters the user has to
enter verbatim, italic denotes placeholders and non-highlighted
pieces are used as a structure: "[]" specifying an optional part,
"{}" a mandatory part, with "|" used for alternations. The "!" for
negation is better supported before the option than after it, too.
The patch makes a few files consistent with this style already used
in manpages.
diff --git a/extensions/libxt_string.man b/extensions/libxt_string.man
index 3f3e5b7..9e3b25c 100644
--- a/extensions/libxt_string.man
+++ b/extensions/libxt_string.man
@@ -1,6 +1,6 @@
 This modules matches a given string by using some pattern matching strategy. It requires a linux kernel >= 2.6.14.
 .TP
-.BI "--algo  " "bm|kmp"
+\fB--algo\fP {\fBbm\fP|\fBkmp\fP}
 Select the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-Morris)
 .TP
 .BI "--from " "offset"