library: adding IO accounting

This is a modification of MR !122 by @renit1609 to fit the new
library.

Problem statement:
The procps library has no PROC_FILLIO flag to
fetch the proc field "/proc/[pid]/io" data
process-wise.
IO Accounting is not included as part of procps.

Requirement:
We have a requirement to fetch process wise
IO utilization which can be used for monitoring.

When looking through the procps library, I see
that IO Accounting (/proc/[pid]/io) is not being
included as part of procps. There is no such
flag like PROC_FILLIO being included in readproc.h .

Solution:
While looking at the implementation done for
other proc fields, I used the spare bits in app code.
I renamed PROC_SPARE_1 as PROC_FILLIO, the spare bit from
PROC_SPARE_* and used it for fetching /proc/[pid]/io
data as part of the procps library similar to other
fields. I moved the PROC_SPARE_* bits each by 1 bit
to retain the spare bits. Meanwhile added the IO fields
in proc_t structure.

References:
 procps-ng/procps!122
 procps-ng/procps#184

proc/pids.h

@@ -78,6 +78,13 @@ enum pids_item {
     PIDS_ID_TGID,           //    s_int        status: Tgid
     PIDS_ID_TID,            //    s_int        from /proc/<pid>/task/<tid>
     PIDS_ID_TPGID,          //    s_int        stat: tty_pgrp
+    PIDS_IO_READ_BYTES,     //    ul_int       io: bytes read
+    PIDS_IO_READ_CHARS,     //    ul_int       io: characters read
+    PIDS_IO_READ_OPS,       //    ul_int       io: read operations
+    PIDS_IO_WRITE_BYTES,    //    ul_int       io: bytes written
+    PIDS_IO_WRITE_CBYTES,   //    ul_int       io: cancelled write bytes
+    PIDS_IO_WRITE_CHARS,    //    ul_int       io: characters written
+    PIDS_IO_WRITE_OPS,      //    ul_int       io: write operations
     PIDS_LXCNAME,           //      str        derived from CGROUP 'lxc.payload'
     PIDS_MEM_CODE,          //   ul_int        derived from MEM_CODE_PGS, as KiB
     PIDS_MEM_CODE_PGS,      //   ul_int        statm: trs