syscalls.2, uname.2, boot.7: Global fix: s/OS/operating system/

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

man2/syscalls.2

@@ -638,7 +638,8 @@ There are many exceptions, however, mostly because
 older system calls were superseded by newer ones,
 and this has been treated somewhat unsystematically.
 On platforms with
-proprietary OS emulation, such as parisc, sparc, sparc64 and alpha,
+proprietary operating-system emulation,
+such as parisc, sparc, sparc64 and alpha,
 there are many additional system calls; mips64 also contains a full
 set of 32-bit system calls.
 

man2/uname.2

@@ -44,8 +44,8 @@ struct utsname {
     char sysname[];    /* Operating system name (e.g., "Linux") */
     char nodename[];   /* Name within "some implementation-defined
                           network" */
-    char release[];    /* OS release (e.g., "2.6.28") */
-    char version[];    /* OS version */
+    char release[];    /* Operating system release (e.g., "2.6.28") */
+    char version[];    /* Operating system version */
     char machine[];    /* Hardware identifier */
 #ifdef _GNU_SOURCE
     char domainname[]; /* NIS or YP domain name */

man7/boot.7

@@ -16,7 +16,7 @@ boot-scripts \- General description of boot sequence
 .LP
 The boot sequence varies in details among systems
 but can be roughly divided to the following steps:
-(i) hardware boot, (ii) OS loader,
+(i) hardware boot, (ii) operating system (OS) loader,
 (iii) kernel startup, (iv) init and inittab,
 (v) boot scripts.
 We will describe each of these in more detail below.