Manual pages: Standardize on ENVIRONMENT as section title

There is quite some value (in terms of readability and user
expectations) if consistent names are used for the sections
within manual pages. This patch is one of a series to bring
about this consistency.

But, let's make sure that the section is consistently titled
across pages. Currently we have ENVIRONMENT (many) or ENVIRONMENT
VARIABLES (3).  Let's standardize on ENVIRONMENT (which is also
what is suggested in man-pages(7)).

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

disk-utils/fsck.8

@@ -345,7 +345,7 @@ with
 Options to different filesystem-specific fsck's are not standardized.
 .SH FILES
 .I /etc/fstab
-.SH ENVIRONMENT VARIABLES
+.SH ENVIRONMENT
 The
 .B fsck
 program's behavior is affected by the following environment variables:

login-utils/sulogin.8

@@ -72,7 +72,7 @@ will wait forever.
 Display help text and exit.
 .IP "\fB\-V\fR, \fB\-\-version\fP"
 Display version information and exit.
-.SH ENVIRONMENT VARIABLES
+.SH ENVIRONMENT
 .B sulogin
 looks for the environment variable
 .B SUSHELL

text-utils/pg.1

@@ -196,7 +196,7 @@ input file or writes on the terminal,
 will immediately display the prompt.
 In all other situations these keys will terminate
 .BR pg .
-.SH ENVIRONMENT VARIABLES
+.SH ENVIRONMENT
 The following environment variables
 affect the behavior of
 .BR pg :